1. Field of the Invention
Embodiments generally relates to an image pickup apparatus and an image pickup method.
2. Description of the Related Art
In an electronic image pickup apparatus, such as a digital still camera, a conventional white balance correction process is a process of uniformly applying a single white balance gain to an entire image.
For example, in a scene with a low level of illumination (such as surrounding light), photographing is performed by emitting a flash to compensate for a lack of light-exposure. When photographing is performed by emitting a flash, a white balance gain is calculated according to a light intensity of the emitted flash. When a scene is captured with both surrounding light and flash light, a subject close to a digital still camera can be captured with a proper white balance. However, a subject far from the digital still camera relies on the surrounding light rather than the flash light. Thus, when a light source of the surrounding light is a fluorescent lamp, a color temperature difference between the surrounding light and the flash light can be large. As a result, when a white balance correction process is applied to an entire image, a white subject far from the digital still camera becomes yellow because an improper white balance occurs.
Japanese Patent Laid-Open Gazette No. 2005-347811 discloses technology for calculating an optimal white balance gain by interpolating a flash image and a non-flash image when an image size of the flash image is different from that of the non-flash image. According to Japanese Patent Laid-Open Gazette No. 2005-347811, the entire flash image and the entire non-flash image are each divided into a plurality of blocks; a white balance gain is calculated by comparing corresponding blocks with each other; and the calculated white balance gain is defined as a white balance gain of a central pixel of a corresponding block. A white balance gain of each non-central pixel is obtained by performing interpolation according to a distance from a central pixel of an adjacent block.
According to Japanese Patent Laid-Open Gazette No. 2005-347811, in a case of a subject having a boundary, such as an edge, if a color temperature is significantly changed in the boundary, an improper white balance gain is derived around the boundary when performing interpolation based on a distance from a central pixel.
Japanese Patent Laid-Open Gazette No. 2002-369211 discloses an interpolation method different from Japanese Patent Laid-Open Gazette No. 2005-347811. According to Japanese Patent Laid-Open Gazette No. 2002-369211, a flash image and a non-flash image are acquired by reducing numbers of pixels through down-sampling, and a color temperature condition is calculated in a unit of a pixel. When the non-flash image is a down-sampled image, a white balance gain is obtained by interpolating down-sampled pixels. A color temperature condition of a pixel to be interpolated is obtained by selecting three pixels close to the pixel to be interpolated and determining a color temperature condition of a pixel having a brightness value approximate to that of the pixel to be interpolated as the color temperature condition of the pixel to be interpolated. A white balance gain of the pixel to be interpolated is obtained from the obtained color temperature condition.
According to Japanese Patent Laid-Open Gazette No. 2002-369211, while interpolation is being properly performed for a subject having a boundary, such as an edge, an occurrence of image streaking in which a white balance gain is frequently changed causes interpolation to be unnaturally performed.
That is, when a non-flash image, such as a live-view image, having a smaller number of pixels than a flash image is used, a white balance gain of a down-sampled pixel is obtained by interpolation. However, a proper white balance gain cannot be obtained by interpolation based on a distance between pixels or using a pixel value of an adjacent pixel as it is.
Therefore, there is a need in the art for an apparatus or a method of providing an entire image with proper white balance.